In many contexts, especially in flue gas cleaning, electrostatic precipitators (ESP) are highly suitable dust collectors. Their design is robust and they are very reliable. Moreover, they are most efficient. Degrees of separation above 99.9% are not unusual. Since, when compared with fabric filters, their operating costs are low and the risk of damage and stoppage owing to functional disorders is considerably smaller, they are a natural choice in many cases. In an electrostatic precipitator, the polluted gas is conducted between electrodes connected to a high-voltage rectifier. Usually, this is a high-voltage transformer with thyristor control on the primary side and a rectifier bridge on the secondary side.
This arrangement is connected to the ordinary AC mains and thus is supplied at a frequency, which is 50 or 60 Hz.
The power control is affected by varying the firing angles of the thyristors. The smaller the firing angle, i.e. the longer conducting period, the more current supplied to the precipitator and the higher the voltage between the electrodes of the precipitator.
Modern power supplies for these ESP are so-called series loaded resonant converters (SLR), which allow to have high-power (typically in the range of 10-200 kW) and high-voltage (50-150 kV DC) while at the same time keeping switching losses at a minimum. The focus of the R&D is higher output power.
The used topology is a series loaded resonant converter, SLR, e.g. as given in FIG. 1. The three phase main with the three phases 1-3 which can be individually switched by switches 4, is rectified by a six-pulse rectifier 6 e.g. comprising diodes 5. This rectifier may however also be an actively switched rectifier. The rectified voltage is smoothed by a DC-link capacitor 13 in the DC-link 6. The DC link voltage is fed to a transistor bridge 8 (H-bridge), comprising four transistors 14, 14′, 15, 15′. The output of the bridge 8 (high frequency AC voltage) is connected, via a resonant tank 9, to the primary of a transformer 10. The resonant tank 9 comprises an inductor 16 and a capacitor 17 in series and together with the primary winding 18 these elements basically define the resonance frequency of the resonant tank, which correspondingly can only reasonably be operated around this resonance frequency. The transformer 10, consisting of the primary winding 18 and the secondary winding 19, adapts the input voltage (mains) to the load 12 (ESP, 50-150 kV). The secondary alternating voltage of the transformer 10 is rectified by a high voltage rectifier 11 and fed to the load 12. The output voltage is normally negative.
In order to increase the power handling capability and to establish a scaleable design, modularizing is used. The fundamental issue when modularizing is to control the load sharing i.e., to secure that different modules take equal or well defined shares of the load.
The high voltage transformer 10 is a key component in the system. Due to the high frequency operation the transformer 10 can be made small. Thus, compared to low frequency designs, the power density is high, leading to high electric fields and to the demand of a high efficiency.
Normally, a transformer for an ESP power supply is immersed in oil to get the appropriate electrical insulation. Also solid insulation materials are in use, in order to strengthen the system where needed.
The presences of high voltage, high frequency AC-fields gives raise to partial discharges (PD) in the insulation system, which will by time deteriorate the system.
The high operation puts focus on the parasitic components of the transformer, these are: leakage inductor, winding capacitor, and magnetizing inductor. The first affect the operation the most. The leakage inductor is connected in series with the resonance tank inductor and has to be considered in the design, as a circuit element. Hence, it has to be controlled and defined by the design.